1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine or a printer for transferring onto a recording material a toner image formed on an image bearing member using, for instance, an electrophotographic process and subsequently fixing the toner image to form a permanent image on the recording material.
2. Related Background Art
Up to now, as color image forming apparatuses capable of outputting a full color image, apparatuses having the following structure have been put in practical use. That is, at a first transferring area formed in a contact part between an image bearing member surface and an intermediate transferring body surface, a first transferring bias is applied to a first transferring member disposed on a rear side of the intermediate transferring body to temporarily transfer onto the intermediate transferring body surface a toner image on the image bearing member surface (hereinafter, referred to as primary transfer). After that, a transferring material passes through a second transferring area formed at a contact part between the intermediate transferring body and a second transferring member to apply a second transferring bias thereto, so that the toner image on the intermediate transferring body surface is transferred again onto the transferring material (hereinafter, referred to as secondary transfer).
FIG. 8 shows an example of the color image forming apparatus having the above structure. Now, referring to FIG. 8, a description will be given of an operation of the image forming apparatus having the above structure.
In the image forming apparatus of this example, image exposure using a laser beam L is applied from an exposure apparatus 103 through a reflection mirror 104 onto a rotation drum type electrophotographic photosensitive member (hereinafter, referred to as photosensitive drum) 101 as the image bearing member rotating in a direction of an arrow R1, which is uniformly charged by a charger 102. Then, latent images corresponding to target color images are formed on an exposure area A.
Next, the latent images are developed by developing devices 105 (yellow developing device 105Y, magenta developing device 105M, cyan developing device 105C, and black developing device 105Bk) to thereby form a yellow toner image, a magenta toner image, a cyan toner image, and a black toner image on the photosensitive drum 101, respectively. The yellow toner image, the magenta toner image, the cyan toner image, and the black toner image are superposed in order on a surface of an intermediate transferring belt 106 at a primary transferring nip part B as a primary transferring area between a primary transferring roller 107 and the photosensitive drum 101 and primarily transferred thereonto. This primary transfer was carried out while the intermediate transferring belt 106 makes four rotations in a direction of an arrow R2. The toner images of full color thus superposed on the intermediate transferring belt 106 are collectively secondarily transferred as the full color toner image corresponding to a target color image onto a transferring material P fed to a secondary transferring nip part C as a secondary transferring area between a secondary transferring roller 108 and a secondary-transferring opposing roller 106b. The transferring material P, after undergoing the secondary transfer, is transported to a fixing device 115 where the toners of four colors are melted for color mixture by applying a pressure and heat thereto and fixed onto the transferring material P. Thus, a full-color final image is formed on the transferring material P.
After the above process is completed, a secondary transfer residual toner on the intermediate transferring belt 106 is removed by an intermediate transferring belt cleaner 109. In addition, a primary transfer residual toner on the photosensitive drum 101 is collected by a cleaner 110 and used for a subsequent cycle.
Note that in such an image forming apparatus, a laser beam scanning direction is called a main scanning direction (direction perpendicular to a direction in which the photosensitive drum moves), whereas the directions of the arrows R1 and R2 in which the photosensitive drum 101 and the intermediate transferring belt 106 rotate, respectively are each called a sub-scanning direction.
For the purpose of further improving an image quality of a final image obtained by the above image forming apparatus, however, the inventors of the present invention have made various studies on the image forming apparatus of such a type and found that, upon primarily transferring the toner image formed on the photosensitive drum 101 surface onto the intermediate transferring belt 106 surface, an abrupt rotation variation of the photosensitive drum 101 may occur, which causes an exposure unevenness of the laser exposure L. This leads subsequently to an image streak occurring on the toner image formed on the photosensitive drum 101 surface.
This is because under such a condition that no toner is within the primary transferring nip part B formed by the photosensitive drum 101 and the intermediate transferring belt 106, when a leading end of the toner image developed onto the photosensitive drum 101 comes into the primary transferring nip part, a frictional force acting on the photosensitive drum 101 surface from the intermediate transferring belt 106 surface abruptly drops.
To cope with the above problem, the following method has been known. That is, in addition to a toner image of an image pattern that the user demands, a minute dot-shaped toner image is formed additionally on the photosensitive drum 101 with a yellow toner or the like. As a result, the rotation variation of the photosensitive drum 101 and the intermediate transferring belt 106 is suppressed to thereby avoid various types of image failure.
For example, in an image forming apparatus disclosed in Japanese Patent Application Laid-Open No. 11-52758, minute dot toner images are formed on the photosensitive drum through a uniform dispersion to prevent a color drift from occurring on the toner image primarily transferred onto the intermediate transferring belt.
Similarly, in the image forming apparatus configured as shown in FIG. 8, such dot toner images are formed to make the photosensitive drum 101 surface and the intermediate transferring belt 106 surface smooth to each other at the primary transferring nip part B. Hence, the frictional force is reduced in advance, thereby making it possible to eliminate the exposure unevenness resulting from the rotation variation and to prevent the image streak occurrence.
However, upon printing on a transferring material such as coat paper, glossy paper, or a glossy film, if the image formation is performed by additionally forming the dot toner images, the additionally formed dot toner images are conspicuous on the transferring material and the transferring material looks yellowish throughout, which causes a problem in that the image quality is degraded in some cases. This is because the transferring material is high in surface smoothness and superior in secondary transferring property. Therefore, the yellow dot toner image is wholly represented on the transferring material, which is primarily transferred onto the intermediate transferring belt 106 to make the photosensitive drum 101 surface and the intermediate transferring belt 106 surface smooth to each other to thereby reduce the frictional force.
Also at the time of printing at a printing speed lower than usual for the purpose of obtaining a high-quality final image on the various types of transferring materials with a fixability, glossiness, and resolution higher than usual, the additionally formed dot toner images are conspicuous on the transferring material, which accordingly looks yellowish throughout. This causes a problem in degraded image quality. This is because the printing speed falls and hence, the fixability as well as the secondary transferring property are improved. Therefore, the yellow dot toner image fixed onto the transferring material supposedly appears with the increased surface glossiness.